Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:

SEDNOID

There are currently only two known sednoids. The first is Sedna (from which the word sednoid is derived). The other is named 2012 VP113 (or “Veep,” as I like to call it).

A possible third sednoid was discovered in late 2015. It has the fun, easy-to-remember name V774104. It may take a while for astronomers to determine V774104’s orbital path. It will take a bit longer for them to think up better names.

There’s a lot of ongoing debate over what exactly these two (or three) objects are. They might be former Kuiper belt objects, or they might be part of the Oort cloud, or they may even be objects captured from other star systems.

For our purposes, the defining characteristic of sednoids is that they keep their distance from the rest of the Solar System, coming no closer to the Sun than 75 AU. For the sake of comparison, Neptune orbits the Sun at a distance of approximately 30 AU, and the Kuiper belt terminates at a distance of about 50 AU.

This means sednoids are so distant that they don’t have any significant gravitational interactions with the eight known planets. As far as Sedna and Veep are aware, there may as well be no planets in the Solar System at all.

Okay, the orbits of both Sedna and Veep are a little too strange. They’re too eccentric. Way too eccentric.

At perihelion (closest approach to the Sun), Veep is approximately 80 AU away; at aphelion (farthest distance from the Sun), Veep is over 400 AU away. Sedna’s orbit is even crazier, with perihelion at 75 AU and aphelion at a distance of over 900 AU!

It’s hard to believe the sednoids ended up in these bizarre orbits on their own, so they must have had gravitational interactions with something. If the eight known planets couldn’t have influenced the sednoids, does that mean there’s another planet out there? Could the elusive and controversial Planet X be responsible for these weird orbits?

Assuming Planet X exists at all.

P.S.: I’ve been highly skeptical of the whole Planet X thing, or as it is now being called the Ninth Planet hypothesis. However, after yesterday’s post on the clustering of scattered disk objects and today’s post on sednoids, I have to admit that something odd seems to be going on beyond Neptune’s orbit.

Dang it, I thought I was finished exploring the Solar System. Now there’s another planet!?! Are we sure this isn’t yet another false alarm over Planet X?

To escape the hype over this alleged ninth planet, I decided to find the original source for this story, a paper entitled “Evidence for a Distant Giant Planet in the Solar System.” It was published on January 20, 2016, in The Astronomical Journal (click here).

First off, this paper does not announce the discovery of a new planet. Instead, it examines a peculiar trend astronomers have noticed in a region of the Solar System called the scattered disk (a region which partially overlaps the Kuiper belt). This odd trend is offered as circumstantial evidence that an extra planet might exist.

For the purposes of today’s post, I’ve named this hypothetical planet “Neo-Pluto.”

If Neo-Pluto is Real…

Billions of years ago, as the Sun shed its first rays of sunlight, a certain number of planets coalesced from the protoplanetary disk. That number was greater than eight. It was also greater than nine. A lot greater.

Like unruly children, these early planets jostled around in their orbits, pushing and pulling each other with their gravity, and sometimes colliding. One such collision led to the formation of Earth’s moon. Another probably knocked Venus into its retrograde rotation, and another may have tipped Uranus sideways.

Jupiter likely formed at the outermost edge of the protoplanetary disk, but it didn’t stay there. As this giant planet migrated inward, it wreaked havoc on the young Solar System. Smaller objects that wandered too close were either gobbled up, adding to Jupiter’s already considerable mass, or they were hurled out of the Solar System by Jupiter’s colossal gravity.

Neo-Pluto was one of those unlucky objects to be ejected from its original orbit, but it was not sent into total exile. It managed to stay within the gravitational influence of the Sun, and today it lingers in the cold depths of space, brooding over its fate.

But Neo-Pluto isn’t exactly a lightweight either. At approximately ten Earth masses, it has considerable gravity of its own, and the effects of that gravity have been noticed in the scattered disk.

Scattered disk objects (SDOs) are… well… scattered. They have highly eccentric (non-circular) and highly inclined (tilted) orbits; however, many known SDOs have almost the same perihelion (point of closest approach to the Sun). That can’t be a coincidence. It seems that just as Jupiter perturbs the orbits of asteroids in the asteroid belt, Neo-Pluto has pushed scattered disk objects into certain orbital trajectories, causing their perihelia to cluster.

Or so it seems. This all assumes that Neo-Pluto exists.

If Neo-Pluto Is NOT Real…

The authors of this “Distant Giant Planet” paper use a hypothetical planet to explain what’s happening in the scattered disk, and they clearly prefer that explanation, but they do acknowledge other possibilities.

Instead of one large planet, there could be a great many smaller planets and/or dwarf planets affecting the scattered disk.

A passing star might have disrupted the scattered disk long ago.

Observational bias may be at work, meaning that SDOs with a specific perihelion might be easier to see from out vantage point here on Earth. It’s unclear why that might happen, but a more thorough survey of the scattered disk could reveal a more random distribution of perihelia.

The best way to prove that Neo-Pluto exists is to find it. Alternatively, we could continue studying the scattered disk to see if this perihelion-clustering trend continues.

Until then, all we can say for certain is that the Solar System has at least eight planets.

P.S.: In part because of this latest new planet “discovery” as well as other rumored “discoveries” in recent months, today Sci-Fi Ideas is reposting my Sciency Words article on “Planet X.” Click here to check that out.